In U.S. Pat. Nos. 4,590,224, 5,151,082, 5,152,743, 5,224,926 and U.S. Pat. No. 5,735,809 there are disclosed apparatus for in vivo separation of plasma from blood utilizing one or more hollow elongated microporous fibers implanted within a patient's blood vessel. The fibers are made of a microporous polymeric fiber membrane material having a pore size sufficient to allow diffusion of plasma into the hollow fiber interior but preventing cellular components larger than plasma to diffuse, ultrafiltrate or enter the fiber interior. The fiber or fibers are implanted within the blood vessel without significantly obstructing fluid flow through the vessel while providing the aforesaid in vivo plasma separation. The fiber assembly is secured in fluid communication with a catheter, preferably a dual lumen catheter having a first tube permitting plasma passage from the fiber and a second tube for returning plasma to the blood vessel after treatment. Various configurations and methods of fabrication as well as materials having a variety of characteristics and performance abilities are disclosed in the aforesaid patents and application as are various systems, apparatus, components and methods for use including measurement of blood parameters, kidney dialysis, and separation and removal of a substantial number of specific materials and plasma components, the descriptions of which are incorporated herein by reference. As efficient as the aforesaid hollow fiber loops are in separating blood plasma, it has been found that the hollow fibers may not provide the most optimum means of in vivo plasma extraction in all applications, treatment protocols, or in all patients.